Sudden chicken death

So about a month or two ago one of my Pekin bantams was broody on some eggs, when they hatched she was a great mother and about after 3-4 weeks of her being the mother she was beginning to get quite tired she would still eat and everything. So we took her away from the chicks and put her with her other Pekin friends. She still seemed to be quite tired though and she had stopped laying eggs while she was broody and after that she had only payed one egg so we thought she would just get better and eventually start laying again we had checked her over a few times no visible signs of disease or problems. So we carried on as normal and in the past month we had entered her in two shows of which she won one and came third in the other. Yesterday I knew there was something wrong so I put her in a cage inside so shed be away from the others and in the warmth and she seemed okay.But today I checked on her a few times and she was ok but I checked on her earlier and she had passed away. The was no blood or foaming to the mouth nod she didn't have mites or anything I also don't think the was in pain because she was always quite calm, she was just under a year old. I'm really sad at the moment as she was my favourite chicken and I'm just wondering if anyone may know what happened to her. Thank you.